This invention relates to an electrofiltering process for dewatering or separating particles from a liquid or colloidal suspension, by the use of an electrofilter wherein the catholyte is recirculated from the cathode to the anode.
Almost every particle occuring in nature has an inherent electrical charge. This charge can be utilized to achieve a separation as in electrophoresis. Electrophoresis is the movement of charged particles in a conductive liquid or colloidal suspension within an electric field. Charge on the particles may be altered by changing the pH, conductivity, temperature, or composition of the solution to be separated. Electroosmosis is the movement of water molecules towards the cathode as a function of an applied electrical force. The rate of movement of the particles or molecules is generally directly proportional to the amount of applied current.
An electrochemical cell is formed by the juxtaposition of electronic and electrolytic conductors so that as electricity is passed from the electronic conductor, such as a metal, to the electrolytic conductor, such as an acid, base, or salt solution, a chemical reaction occurs at their interface. The electronic-electrolytic interface at which oxidation occurs is known as the anode or positive electrode and that at which reduction occurs is known as the cathode or negative electrode.
When charged particles are placed between the anode and cathode of the electrochemical cell, the particles will tend to migrate towards an electrode as a function of their charge. This process is called electrophoresis. For example, kaolin clay particles are negatively charged so they migrate towards the anode. Accordingly, a kaolin clay suspension can be "dewatered" by placing the clay suspension within an electrochemical cell and applying a direct current. The clay particles settle or deposit on the anode surface and on each other, displacing water or liquid molecules electroosmotically to form a more dense cake layer or slurry with a higher solids content than that of the liquid or aqueous suspension. The anode is removed from the cell and the dense layer of clay particles is recovered by removing the layer from the anode. The water is collected or removed at the cathode.
Since the rate of migration of the particles is a function of current density, it would seem desirable to apply as much current as possible. However, since the generation of acid (H.sup.+) and base (OH.sup.-) by hydrolysis of water molecules and the amount of oxidation at the anode is proportional to the amount of current, the applied current must be limited to reduce these reactions which would otherwise shorten the electrode life.
An example of a dewatering system utilizing the method of placing a suspension between electrodes and applying an electrical current is described by U.S. Pat. No. 4,367,132 to Bell et al. In this method, dewatering of chemically precipitated sludge is achieved by passing direct electric current through the sludge between a pair of submerged perforated electrodes. As a result of this treatment, the liquid phase of the sludge flows by electroosmosis towards the cathode where it is collected after passing through the perforated cathode. Additional liquid is accumulated at the perforated anode where it diffuses as the sludge solids accumulate on the anode. Since the anode may be consumed in the process, the perforated electrodes used in this method are relatively simple metal sheets preferably constructed of low cost materials such as iron, aluminum or graphite. The electrodes are optionally covered with a liquid porous, non-conductive membrane made of a material such as polypropylene or rayon fabric to prevent the sludge material from clogging the electrode. (See column 5, lines 5-27 of the '132 patent.)
In an important commercial application of electrofiltration, as applied to the separation of kaolin clay particles, it has been found that a titanium electrode, in the form of a sheet to which an expensive protective coating of an acid and oxidation/resistant conductive metal is applied, is the only electrode that is practical. This electrode is very expensive because it is formed of titanium and the protective coating of acid and oxidation resistant metal costs about $100 per square foot. The function of the coating is, of course, to protect the titanium backbone from the corrosive environment of the electrofilter. In practice, however, the protective coating material does slowly erode, revealing the titanium backbone which then erodes. The replacement cost of these electrodes is substantial. It is therefore desirable to decrease the corrosiveness of the electrode environment.
Another problem which arises with the last-described apparatus is the eventual intrusion of ions, such as H.sup.+ and OH.sup.-, generated respectively at the positive and negative electrodes, into the electrolyte flowing into the working space. The presence of these ions in the working space alters the conductivity of the electrolyte and diffuses into the kaolin clay cake. In particular, movement of H.sup.+ ions from the anode towards the cathode causes a problem with acid "floccing" of the kaolin clay which results in the formation of an acid cake.
One solution in the prior art to this problem was to continually replenish the anolyte to remove the reaction products such as OH.sup.- and H.sup.+. the continual replenishment of anolyte adds operating difficulties and expense to the operation.
Both the anode element and the anolyte are usually contained within an ion-selective membrane shell so that the OH.sup.- ions from the hydrolyzed water may not travel from the cathode back to the anode and the liquid in the anolyte is prevented from exiting the anode assembly. It is critical to choose the membrane carefully to accomplish such selective ion transport at the anode assembly. Even with great care in the selection, however, the ion-selective membrane forming the outer compartment of the anode assembly is not a perfect barrier for retaining the anolytic solution added into it, so that some of the anolytic ions migrate through the membrane wall into the liquid or aqueous suspension. Often, the anolytic ions become trapped within the cake or slurry layer and remain there which can have deleterious effects on the filtered product.
It is therefore an object of the present invention to provide an efficient and inexpensive process for the dewatering or separation of particles from a liquid or colloidal suspension which produces a superior filtered product.
Accordingly, it is an object of the present invention to provide an electrofiltering process wherein the electrolyte is recirculated to neutralize reaction products and decrease the need for readjustments of the pH and conductivity of the electrolyte.
Still a further object of the present invention is to provide an electrofiltering process wherein the catholyte is recirculated into the anolyte to avoid acid precipitation of the separated solids and thereby improve the nature of the product collected at the electrode assembly.